Boosting Nitrogen Activation via Bimetallic Organic Frameworks for Photocatalytic Ammonia Synthesis

Abstract

Photocatalytic ammonia synthesis from N2 is a carbon-neutral strategy, although its efficiency is impeded by the activation of inert NN triple bonds. In N2 activation, the electron acceptance process is often strongly coupled with the electron donation process, leading to a high potential activation energy barrier and low photocatalytic activity. Herein, we proposed a strategy to decouple these two processes by bimetallic organic frameworks (BMOFs) for boosting N2 activation. The rationally designed BMOFs are composed of two functional metal nodes, in which the hard acid metal node with a high ionization potential (I n) accepts the electron from N2 and the soft acid metal node with a low I n donates the electron to N2. Owing to the bimetal synergistic effect, the potential activation energy barrier of N2 is reduced, as confirmed by the in situ Fourier transform infrared (FTIR) spectra and density functional theory (DFT) calculations. Via testing six kinds of bimetal combinations, it is found that, as the ionization potential difference (ΔI n) between the two metals is ≥6 eV and the proportion of high I n metal reaches ∼20%, the bimetal synergistic effect becomes dominant. In all the as-prepared BMOFs, the optimal BMOF­(Sr)–0.2Fe photocatalyst exhibits an NH3 evolution rate up to 780 μmol g–1 h–1. This work may unveil a corner of the hidden mechanism for the chemical bond activation in a broad range of catalytic processes.

Unveiling the Synergy of O‐Vacancy and Heterostructure over MoO3‐x/MXene for N2 Electroreduction to NH3

“…In addition, a new NH 3 peak (≈1215 cm −1 ) is emerged on MoO 3‐ x but absent on MoO 3‐ x /MXene. [ 58 ] This suggests that the produced NH 3 can be easily desorbed from the surface of MoO 3‐ x /MXene, favorable for a quick release of the active sites for new reactions, while the strong NH 3 adsorption on MoO 3‐ x causes the difficulty in NH 3 desorption and a slow process in the catalytic cycle. The more favorable NH 3 desorption on MoO 3‐ x /MXene relative to MoO 3‐ x can also be proved by the MD simulations (Figure S39, Supporting Information).…”

Recent advances in bimetallic metal–organic frameworks (BMOFs): synthesis, applications and challenges

“…In addition, the synergistic effect between two heterogeneous metals may also affect the photocatalytic process of bimetallic MOFs to some extent. Zhao and co-workers 88 proposed a strategy to decouple the electron acceptance and transfer processes using a bimetallic organic framework to facilitate the activation of N 2 (Fig. 14).…”

“…In addition, the inter-space between sheets effectively alleviated the volume change of the MOFs during OH À insertion and extraction. 88 However, to assess whether the introduced ions have an optimal ion concentration to increase the capacitance and conductivity of MOFs, Chen and co-workers prepared a bimetallic Co/Ni organic backbone material (Co/Ni-MOF) via a one-step solvothermal method (Fig. 16a).…”

Modular assembly of electron transfer pathways in bimetallic MOFs for photocatalytic ammonia synthesis

“…Recently, benefited from the modular property, uniform pore structure and well crystal texture, metal organic frame materials (MOFs) have become an ideal platform for the rational design of high-efficiency N 2 fixation photocatalysts. [7][8][9][10] For example, Huang et al achieved a photocatalytic nitrogen fixation efficiency at a rate of 12.3 μmol g −1 h −1 by amino-functionalizing the organic ligand of MIL-125 (Ti). 11 Introducing the rare-earth Ce element into the metal cluster nodes of MOF-76, Zhang et al simulated the biogenic π backdonation mechanism to obtain a nitrogen fixation rate of 34 μmol g −1 h −1 at room temperature.…”